Fixed nfc-extras build files

The com.android.nfc_extras target was including the test class.
The nfc-extras/tests/Android.mk file was not being included so
was not being built by continuous build and could not be built
using make NfcExtrasTests.

Bug: 38487370
Bug: 30188076
Test: make checkbuild
Change-Id: I17f6723b899f26de329a2e74177bd02ba864b7b2